Engine air to fuel ratio cylinder imbalance diagnostic

Abstract

A diagnostic for identifying cylinder to cylinder air/fuel ratio faults of an engine having closed loop fuel control. Air mass flow is accumulated for a plurality of load bands on the engine load/speed map, and for each load band a rich/lean air fuel ratio is determined at a mass threshold. This threshold data is processed and compared with fixed data to determine whether any cylinder of an engine is experiencing an air/fuel ratio fault which is substantially different to the remainder.

The invention claimed is: 
     
       1. A method of identifying cylinder to cylinder air/fuel ratio asymmetry of a multi-cylinder internal combustion engine having an upstream exhaust gas oxygen sensor and a downstream exhaust gas sensor, the method comprising the steps of:
 selecting a plurality of successive load bands on an engine load/speed map; 
 determining the outputs of said downstream sensor as lean or rich; 
 for each of the load bands, recording in a respective register a measure of air flow for which the downstream sensor output indicates lean, and for which the downstream sensor output indicates rich; 
 for each of the load bands, determining a cumulative measure of air flow, and at a threshold determining the lean/rich air flow ratio, and a predicted error from said air flow ratio; 
 determining for each of the load bands an average difference in the air/fuel ratio indicated by the outputs of upstream and downstream oxygen sensors; 
 obtaining a predicted difference based on said predicted error and said average difference; and 
 comparing said predicted difference against pass/fail criteria. 
 
     
     
       2. The method of  claim 1 , wherein a plurality of said predicted differences are combined to determine the range thereof, and said range is compared with pass/fail criteria. 
     
     
       3. The method of  claim 1 , wherein the recorded measure of air flow is mass. 
     
     
       4. The method of  claim 1 , wherein the same threshold is applied for each of the load bands. 
     
     
       5. The method of  claim 1 , wherein the content of each register is retained in EEP memory. 
     
     
       6. The method of  claim 5 , wherein the cumulative measure of air flow for each of the load bands is retained in EEP memory. 
     
     
       7. The method of  claim 1 , wherein each register is zeroed after determination of a lean/rich air flow ratio at a threshold. 
     
     
       8. A method according to  claim 1 , wherein the lowest of the load bands commences at a predetermined minimum load. 
     
     
       9. A method according to  claim 1 , wherein the highest of the load bands terminates at a predetermined termination load, and wherein the termination load is less than maximum load. 
     
     
       10. A method according to  claim 1 , wherein said load bands encompass a continuous load range. 
     
     
       11. A method according to  claim 1 , wherein each of said load bands is a substantially equal sub-division of the load range. 
     
     
       12. A control unit for an internal combustion engine operable in accordance with the method of  claim 1 . 
     
     
       13. A vehicle having an internal combustion engine, an electronic control unit, an upstream exhaust oxygen sensor and a downstream oxygen exhaust sensor, the electronic control unit being operable in accordance with the method of  claim 1 .